[dev.typeparams] cmd/compile: fix -G=3 handling of blank methods

Fixes "GO_GCFLAGS=-G=3 go run run.go -- blank.go interface/fail.go".

Change-Id: I669ab06ae29366ce96e2948c89a5c1620afd53db
Reviewed-on: https://go-review.googlesource.com/c/go/+/286214


Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Matthew Dempsky <mdempsky@google.com>
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>
Reviewed-by: Dan Scales <danscales@google.com>